Field of the invention:
This invention relates to belts which show high resistance to flex fatigue and have good length stability.
Prior art
In general, in the field of power transmission belt cords made of polyethleneterephthalate fiber are used widely as load carriers because such cords are high in modulus of elasticity and low in contractility and show high resistance to flex fatigue.
The recent tendency is that power transmission belts are used under severe conditions and those having higher resistance to flex fatigue have been demanded. For example, in the field of motor cars, from the energy saving point of view lightening of weight and space saving of motor cars are in progress. For this purpose, the diameter of pulley to drive the power transmission belt which is the means of transmitting power from a driving shaft to an accessory drive of engine tends to be smaller so as to make the weight of an engine lighter. This tendency, however, involves higher flexing rates of belt and more fatigue of load carriers.
In the case where power is transmitted to a plurality of accessory drives, it was usual to provide a transmission belt for each accessory drive but the recent practice is to adopt the so-called multispindle transmitting system (to drive a plurality of accessory drives by a single belt) for space saving. Here, too, the diameter of pulley has been made smaller and this has brought more flex fatigue of load carriers of belt and earlier breakage of belt.
In order to improve the resistance to flex fatigue of power transmission belt having cords of polyethyleneterephthalate fiber as load carriers, various measures have been taken, such as (1) to increase the number of twist of a cord (Rubber Chemistry and Technology, 42 (1) 159, 1969), (2) to make the diameter of a cord smaller, (3) to make the ultimate viscosity larger (Japanese Patent Application Laying Open Gazette No. 57-201703), and so forth.
However, it is true that the measure (1) above improves resistance to flex fatigue but the modulus of elasticity of belt becomes low, the creep rate shows an increase and the residual elongation becomes larger. These drawbacks involve elongation of belt during use with resultant slip of belt and reduction of transmission ability. The measure (2) also improves resistance to flex fatigue but the required strength cannot be obtained in some cases. The measure (3) improves the resistance to flex fatigue to some extent but impairs the length stability of belt due to high percentage of shrinkage at dry heating. In this connection, polyethyleneterephathalate fiber which possesses high ultimate viscosity and shows a small percentage of shrinkage at dry heating is well known but load carriers made of such fiber are low in modulus of elasticity and show more slip.
As stated above, the conventional power transmission belt uses cords of polyethyleneterephthalate fiber as load carriers but such power transmission belt shows poor resistance to flex fatigue in use under severe conditions in recent years, especially under the present trend of smaller diameter of power transmission belt and under the existence of external force to curve the power transmission belt in the direction contrary to the direction in which the pulley is curved (hereinafter referred to as "reverse bend"), and therefore it is damaged and breaks in a short period time.